A hot gas injection lance (26) for injecting hot gas into a vessel (11) is made of three modules (26A, 26B and 26C) which all fabricated separately and brought together in successive steps and connected together by releasable fastenings. Lance module (26A) is a main duct module providing an elongate duct (31) through which to direct hot gas into an upper region of the vessel. Lance module (26B) is a gas inlet module through which to direct hot gas into duct (31) of module (26A). Lance module (26C) is a central module which includes an elongate central tubular structure (33) that extends within the gas flow duct (31) and carries at its lower end a series of swirl imparting tubular structure (33) that extends within the gas flow duct (31) and carries at its lower end a series of swirl imparting vanes (34) for imparting swirl to the gas flow exiting the duct. Main duct module (26A) has a mounting flange (110) that abuts a flange (122) on vessel (11) and the flanges (110, 122) are fastened together by bolts (121). Gas inlet module has a lower mounting flange 111 which abuts a flange (103) at the upper end of module (26A) and the flanges (111, 103) are connected together by bolts (112). Central module (26C) has a mounting flange (115) that abuts a flange (114) at the upper end of inlet module (26B) and the flanges (114, 115) are fastened together by bolts (116).